1. Field of the Invention
The present invention is in the field of solid-state electronic circuits, and, more particularly, in the field of digital-to-analog converters using current sources.
2. Description of the Related Art
During monolithic integration of high-accuracy current sources for volume production of solid-state circuits, the problem arises that, because of the manufacturing variations of the current sources, the currents delivered by the latter are not exactly equal to one another. The current sources are implemented, for example, with the collector/emitter paths of bipolar transistors or the source/drain paths of, preferably insulated-gate, field-effect transistors of a so-called current bank, with the base or gate terminals, respectively, connected to a common constant potential.
According to Rudy van de Plassche, "Dynamic element matching puts trimless converters on chip," Electronics, June 16, 1983, pp. 130-134, and R.J.v.d. Plassche, "Monolithic 14-bit DAC with 85 dB S/N ratio," ELECTRONIC COMPONENTS AND APPLICATIONS, Vol. 2, No. 4, August 1980, pp. 235-241, the current inaccuracy of a 14-bit monolithic integrated digital-to-analog converter ("D/A converter") with bipolar transistors is largely compensated for by the use of a current divider and by switching the currents of a plurality of current sources to three current paths in a "rotating" manner by means of a cyclic shift register, with the first and second current paths being fed half the current of the third path. By cascading a plurality of such current dividers, a high-accuracy monolithic integrated D/A converter can be implemented.
The invention makes use of this principle of "dynamic element matching," as it is called in the above two references, but does not use the above-mentioned current divider of the prior art D/A converter. Rather, the present invention returns to the idea described in Applicant's non-prepublished European Patent Application No. 87 10 3742.0 (corresponding to U.S. patent application Ser. No. 07/074,205). Rather than using a binary signal to drive the "rotating" current sources, the binary signal is changed by means of a code converter into another coded binary signal which has a continuous sequence of states of the same kind. The instantaneous number of the states of the same kind is equal to the instantaneous value of the binary signal. The coded binary signal is caused to "rotate" by means of cyclic shift register, and the "rotating" signal drives a set of switches to turn the individual current sources on or off, or to switch them over.
The frequency of the shift signal for the shift register must be at least (2.sup.n -1) times higher than the frequency of the sampling signal at whose pulse repetition rate the binary signals occur. However, such a shift signal is not always available, e.g., because an oscillator of correspondingly stable frequency is too expensive for a specific application. In addition, the frequency of the shift signal often lies in ranges which necessitate a particularly careful design of, and specific manufacturing processes for, the integrated circuit.